What is antigen What is epitope & paratope? Classification of antigen Pro antigen Superantigens Antigenicity Determinants of antigenicity Test for antigen detection

1. ANTIGEN
VIDHUN.T
BSC MLT INTERN
DR.M V SHETTY COLLAGE

2. 1 What is antigen?
2 What is epitopes & paratope?
3 Classification of antigen
4 Pro antigen
5 Superantigens
6 Antigenicity
7 Determinants of antigenicity
8 Test for antigen detection

3. WHAT IS ANTIGEN?
 Antigen is a substance that triggers an immune
response in the body, leading to the production of
antibodies. Which it reacts in an observable manner.
 Antigen cam be foreign substances like bacteria,
viruses, toxins as well as non-infectious substances
like proteins, cells.
 They can cause allergic reactions/disease
 Immune system recognises antigen as either
harmful/harmless and responds accordingly to
protected the body.
 Each antigen has a distinct surface feature or epitope

4. WHAT IS EPITOPE & PARATOPE?
 Epitope(Antigenic determinant) is the region on the antigen which
is actually responsible for the antigenic properties of the antigen
 Paratope -Antigen binding site
The part of an antibody which recognizes and binds to an antigen.

5. What is the difference between antigen
and epitope?
 While the antigen evokes the antibody response in
the host, the antibody doesn’t bind to the entire
protein, but only to that segment called the
epitope.

6. CLASSIFICATION
Based on the origin
a) exogenous antigen.
b) endogenous antigen
c) auto antigens
Based on the Immunogenicity
a) complete Antigen
b) incomplete Antigen (haptens)
a)Iso antigens
b)Heterophile Antigens.
Based on the biological classes of antigen
a)T cell dependent (TD)
b)T cell independent (TI)

7. BASED ON THE ORIGIN
a)Exogenous antigen
Exogenous antigens are antigens that have entered the body
from the outside, for example by inhalation, ingestion or
injection.
b)Endogenous antigen
Endogenous antigens are antigens that have been
generated within previously normal cells as a result of
normal cell metabolism or because of viral
intracellular bacterial infection.

8. c)Auto antigens
Auto antigen is usually a normal protein/complex
proteins (and sometimes DNA/RNA) that is
recognized by the immune system of patients
suffering from a specific auto immune disease.
These antigens should be under normal condition
not be the target of the Immune system. But due
to mainly genetic and environmental factors, the
normal immunological tolerance for such an
antigen has been lost in these patients.
Two types
a)Iso antigens
b)Heterophile Antigens.

9. a)Iso antigens
Iso antigens that exist within a species and vary slightly among
individuals.
These antigens play a crucial role in immune recognition and
compatibility.
Blood group antigens like ABO and Rh system are the common
examples.
b)Heterophile Antigens
same/closely related antigens present in different tissues of
more than one species are known as Heterophile antigens.
Antibodies to these antigens produced by one species cross
react with antigens of other species.
Eg: well felix reaction.

10. Based on the immunogenicity
a)Complete antigen
Substance which can induce antibody formation by themselves and
can react specifically with these antibodies.
Eg: proteins, such as those found on the surface of bacteria, viruses
and fungi.
b)Incomplete antigen (haptens)
Substances unable to induce antibody formation on its own but con
become immunogenic when covalently linked to proteins. Called
carriers proteins.
These Antibodies are produced not only against the hapten but also
against the carrier proteins.
Haptens are 2 types

11. Haptens are 2 types
A) Simple haptens
 Combine with specific
antibodies but no Precipitate
is produced
 This is due to univalent
character of Simple haptens.
 Examples – certain drugs,
metals like nickel, small
organic molecules like
penicillin.
B)Complex haptens
 Combine with specific
antibodies to form
Precipitates.
 Complete haptens are poly
valent.
 Examples –chemical like
poison ivy compounds,
capsular polysaccharide of
pneumococci.

12. Based on the biological classes of antigen
Antibody production is the property of B lymphocytes.
For the full expression of this function, however, the cooperation of T
lymphocytes is necessary.
Depending on their ability to induce antibody formation, antigens are
classified as T cell dependent (TD) and T cell independent (TI) antigens.

13. a) T cell independent (TI)
Some antigens can directly
stimulate antibody production by
B cells, without the apparent
participation of T cells. Such
antigens are called TI antigens.
b)T cell dependent (TD)
Others that require T cell
participation to generate an
immune response are called TD
antigens. Several important
differences exist between TI and
TD antigens.

14. T cell independent (TI) antigens & T cell dependent (TD) antigen
 TI antigens are structurally simple,
 Their immune response is critically
dose dependent. Too little is
nonimmunogenic, while too much
results in immunological tolerance
rather than immunity.
 Their antibody response is usually
limited to IgM and IgG3.
 They do not show immunological
memory.
 TI antigens do not appear to require
preliminary processing by
macrophages.
 They are metabolised very slowly
and remain in the body for long
periods.
 TD antigens are structurally
more complex ,
 They are immunogenic over a
wide dose range and do not
cause tolerance readily.
 They induce the full gamut of
immunoglobulin isotypes IgM,
IgG, IgA and IgE.
 They show immunological
memory
 They require preliminary
processing
 They are rapidly metabolized in
the body. se tolerance readily.
T cell independent (TI) antigens T cell dependent (TD) antigen

15. Pro antigen
Low molecular weight substance
which do not induce antibody
formation but can cause delayed
hypersensitivity reaction.

16. SUPERANTIGENS
When the immune system encounters a
conventional T-dependent antigen, only a
small fraction of the T cell population is
able to recognize the antigen and become
activated. However, some antigens can
polyclonally activate a large fraction of
the T cells, setting off massive immune
response. These antigens are called
Superantigens.
Superantigens stimulate up to 10% of T
cells to respond whereas antigen would
normally stimulate only 0.001-0.01% of T
cells to respond

17. example of superantigen
• Staphylococcal enterotoxins
• Staphylococcal toxic shock toxin (TSST-1)
• Streptococcal pyrogenic exotoxins (exotoxin A and
exotoxin B)
• Mouse mammary tumor virus (retrovirus), which causes
breast cancer in mice, is also known to produce
superantigen

18. TYPES OF SUPERANTIGENS
 Exogenous SAGs: soluble proteins secreted by bacteria
and a variety of exotoxins (e.g., SE, SPE, and TSST).
 Endogenous SAGs: cell membrane protein encoded
certain viruses that infect mammalian cells (e.g. MMTV,
and EBV).
 B-cell SAGs: These SAGs stimulate predominantly B
cells. Formation of immune complexes.

19. MECHANISM OF ACTION
 Binding to MHC class II.
 Binding to T cell receptor.
 T cell signaling.

20. ANTIGENICITY
Power of an antigen to induce a immune
response

21. Determinants o antigenicity
 1.Foreignness
 2.Size
 3.Chemical nature
 4. susceptibility to tissue enzyme
 5.Antigenic specificity
 6.Species specificity
 7.Isospecificity
 8.Auto specificity
 9.Organspecificity
 10.Heterogenic specificity

22. 1.Foreignness
Antigen must be foreignness to immune system. According to
Burnnet's clone selection theory, foreignness (non-self) means
substances that never contact with lymphocytes during embryo
period.
2.Size
Larger molecules are highly antigenic.
Lower molecular weight are either non antigenic or weakly antigenic.
3.Chemical Nature
Most naturally occurring antigens are either proteins or
polysaccharides.
Lipids and nucleic acids are less antigenic on their own but do so
when combined with proteins.
4. Susceptibility to tissue enzymes
Substances which can be metabolised and are able to do the action of
tissue enzyme behave as antigen.

23. 5.Antigenic specificity
It depends upon epitope Position of epitope in the
antigen molecule is important for specificity.
6.Species specificity
Tissue of all individual in species possess species specific
antigens.
7.Isospecificity
It depends on isoantigens which may be found in some
but not all members of species.
8.Autospecificity
Self antigens are generally non-antigenic but in some
case such as lens protein and sperm these are not
recognised as self antigen because they are absent
during the embryonic life and develop later

24. 9.Organspecificity
Some organs such as the brain, kidney and
lens protein of different species share the
same antigens, such antigens are the
characteristics of an organ or tissue found
in different species and they are known as
organ specific antigen
10.Heterogenic specificity
The same or closely related antigen may
some time occur in different biological
species, classes and kingdom are know as
heterophile antigens. Antibodies to these
antigen produced by one species cross
react with antigen of other species. Eg.
Forssman.

25. TESTS FOR ANTIGEN DETECTION
✓ Direct ELISA
✓ Direct Immunofluoresence
✓ RIA
✓ Neutralization test CFT
✓ Immunohistochemistry